1. Field of the Invention
The present invention relates to the art of high purity glass and glass optical fiber for transmitting light wave signals, and more particularly to the art of fabricating fused silica optical waveguides and glass preforms.
2. Description of the Prior Art
In the past, optical fibers of fused silica, SiO2, have been formed from preforms in which the relationship of the relatively higher index of refraction of the core to a relatively lower index of refraction of the cladding is predetermined so that when the preform is drawn into a fiber, the fiber will conduct light at predetermined wavelengths, either in single mode or in multi-mode form. The performance of fused silica optical fibers is determined by the amount of attenuation or loss, and by dispersion. Attenuation is presently considered to be the result of absorption and scattering, both of which may be the result of irregularities or imperfections in the formation of the fiber, or of its preform. Dispersion is the result of changes in the refractive indices with wavelength.
For optical fibers to be effective, the attenuation must be minimized. The elimination of attenuation, or loss, has been achieved in the past for the most part by eliminating the impurities in the fused silica, except for the dopant or dopants necessary to control the differential in the indices of refraction between the core and the cladding. Even for the dopant or dopants, a high degree of purity has been sought to achieve sharp fiber profiles and performance.
It was known early that one of the impurities in silica that absorbed light, and thus caused higher attenuation or loss, was the hydroxyl ion (Off). Various methods and processes for reducing the hydroxyl ion were taught, among them the reduction of hydroxyl ions in the silica in the preform""s formation stage by fluorination. See, for example, U.S. Pat. No. 4,579,571 to J. W. Hicks, Jr., which well sets forth the benefits of expelling the hydroxyl ions from the preforms formed by flame hydrolysis by drying the preforms by adding fluorine not only as a drying agent, but also to reduce the refractive index of the cladding.
Fluorination of the silica can be accomplished by depositing silica soot onto a start rod, and placing the start rod with the deposited silica soot in a space or zone, sometimes called a muffle, of a furnace which is made rich in fluorine. Sometimes, the pressure in the space, zone or area of the muffle in which the silica preform is sintered, is raised and the temperature is elevated so that the fluorine in the atmosphere is coerced into the interstices of the silica to expel the hydroxyl ions and to dope the silica with the fluorine. In all of these methods, one of the objects, and often the primary object is the reduction of the refractive index of the cladding glass after sintering and secondarily reduction of the hydroxyl ions by fluorination.
One of the ever present problems in fluorination is the very high reactivity of fluorine with almost all materials. Attempts have been made in the past to use various materials for the structure of the furnace in which fluorination takes place. Most workers in the art have returned to using fused silica for the composition of that part of the furnace structure that faces the interior of the muffle space or zone.
Silica, however, dissipates in the face of the very active fluorine in the fluorinating processes. The silica structure, consequently, has to be replaced after only a few uses of the muffle or muffle tube, or alternatively the muffle tube becomes a part of the preform. These unhappy and expensive results are described, for example, in Abe, U.S. Pat. No. 4,643,751. The problems are exacerbated when such a silica wall surrounding the reaction zone is broached, and the highly reactive hydrogen fluoride (HF) escapes. In an effort to reduce the corrosion due to fluorine, chemical forms of fluorine that are inert at room temperatures, such as, for example sulfur hexafluoride are used to supply the fluorine. Reactive fluorine results; however, from the high temperature dissociation of sulfur hexafluoride (SF6), and forms hydrogen fluoride from the interaction of the fluorine with the hydrogen from the OHxe2x88x921 ions imbedded in the soot. Abe, cited above, made the silica muffle tube become part of the preform.
Efforts have been made in the past to realize a substitute for the silica muffle tube in the drying, fluorinating and sintering furnace. For example, one attempt was made to substitute alumina as the composition for the furnace wall in the muffle. However, it was reported that the fluorine so reacted with the alumina muffle that aluminum and possibly other impurities were imparted into the silica preforms. See, for example, Berkey, U.S. Pat. No. 4,629,485, where such an attempt to substitute alumina resulted in a thick devitrified surface layer on the preform that rendered the preform useless.
Aluminum oxide, however, if it could be used, would last substantially longer than silica in the highly reactive environment of fluorine, and consequently it remains a highly sought objective to make at least the interior surface of the muffle of alumina. However, it is an objective to have an alumina surfaced muffle in the drying, fluorinating and sintering furnace where the glass optical preform and/or fiber resulting from its use does not have impurities from the alumina go into the preform or fiber. Further, it is highly desired in any use of a substitute for the silica composition, that no devitrified surface layer forms on the preforms that would render the preform useless.
In brief, in accordance with one aspect of the present invention, a silica soot is deposited on a start rod or core rod, which is then lowered into a muffle or zone of a furnace in which the atmosphere is maintained with highly reactive fluorine. The muffle or zone in the furnace has an interior facing surface which is composed of a high purity aluminum oxide. The purity is of the order of-one or less parts per million of sodium, calcium, potassium, iron and titanium, with an alumina (Al2O3) content of 99.97%. The temperature is raised to a level suitable for drying and removing water from the soot deposited core rod. The core rod having the soot formed on it may be dried separately in a fluorine or chlorine atmosphere maintained in the muffle or zone. The fluorine may continue to dope into the soot cladding. The temperature in the muffle or zone is raised again to a predetermined level so that the fluorine is doped into the deposited soot. The temperature is raised yet again to another level for sintering and consolidating the core rod with soot preform into a glass.
These and other novel aspects of the invention, together with other aspects thereof, can be better understood by the following description of the preferred embodiments, which are designed to be read in conjunction and together with the accompanying drawings.